1. Field of the Invention
The present invention relates generally to prosthetic devices for replacing injured or diseased natural members and, more particularly, to such devices provided with porous surfaces for promoting bone ingrowth fixation.
Although the invention need not be so limited, it is disclosed in relation to a hip prosthesis adapted for insertion into the upper medullary canal in the femur of a patient. The hip prosthesis of this invention is of the type generally characterized as including a head or ball member, a shaft or stem member for insertion into the intramedullary canal, and a neck member connecting the head and stem. The prosthesis also includes a porous metal surface portion which provides for stabilization by bone ingrowth fixation without requiring any cement.
2. Description of the Prior Art
Prosthetic devices are used to partially or completely replace joints or bone segments in the skeletal structure of humans or animals. One of the major problems involved in the use of prosthetic devices is the attachment of the prosthetic implant to the adjacent bone. There are four general methods by means of which the device can be attached to the bone: (1) force fitting the implant into the medullary canal of the bone; (2) securing the implant in the bone with the aid of screws or pins; (3) bonding the implant by use of a plastic methyl methacrylic resin which is polymerized "in situ"; and (4) employing in conjunction with the implant a porous material into which bone may grow.
In modern times, the original hip replacement was described by Austin-Moore in The Journal of Bone and Joint Surgery in 1954. This original implant did not require cement and instead utilized large fenestrations in bone, the purpose of which was for bone ingrowth for stability. As this fenestration was too large to be ingrown in a practical time interval, the implant was commonly loose and moving within the bone marrow cavity. Clinical symptoms which this produced included pain and limping. Subsequently, after a variety of attempts at force-fitting the implant to the bone and attaching the implant to the bone, the next evolutionary improvement was the use of methyl-methacrylate cement for adhering the implant. Acrylic cements proved to be an improvement over the original cementless implants and were the state of the art for many years until follow-up results showed late loosening, bone loss, and a painful limp requiring very difficult revision surgery. Patients were restricted in terms of their activity level, and young people with arthritis were excluded from this surgery altogether.
A separate scientific enquiry based on the cementless biologically fixed Austin-Moore implant continued, even as cemented implants remained state of the art. A quantum improvement was the development of sintered porous bead technology. This type of porous surface was different than the original cementless implants of Austin-Moore, insofar as the pore size was microscopic and measured in microns, as opposed to the original fenestrations with a large pore size measured in terms of cubic centimeters. The smaller pore size allowed for more rapid and stable bone ingrowth. Originally, surgeons were incredulous that osteo-integration within these microscopic pores could occur. When this was proven, concern changed to various other potential concerns, including potential stress-shielding and optimal pore size for ingrowth. There were other concerns as to which metal alloy had optimal biocompatability. As time passed, various other proprietary techniques to develop porous surfaces were devised, and there were concerns about these, including plasma spray and fiber metals.
These latter concerns continue to this day with the latest proprietary technologies which involve the use of hydroxyappatite coatings. By no means, however, have these technologies assumed the mantle "state of the art."
The AML implant manufactured by DePuy, Inc. of Warsaw, Ind. has been studied in various forms, including different porous sizes, extent of porous coating, and the like. In fact, the AML implant may be the most carefully detailed and thoroughly studied total hip implant to date. It remains state of the art with the best clinical results for the longest time interval. The present invention seeks to improve on the successes of the AML implant and addresses some of the concerns that surgeons have about the short and long-term results of these most successful of the known implants.
It is only in recent years, then, that prosthetic devices have been provided with porous surfaces for bone ingrowth fixation. An excellent example of such a surface results from application of the proprietary porous metal coating of DePuy, Inc. of Warsaw, Ind., provided under the trademark "POROCOAT". Representative patents in this field include U.S. Pat. No. 3,605,123 to Hahn, U.S. Pat. No. 3,855,638 to Pilliar, U.S. Pat. No. 4,536,894 to Galante et al. and U.S. Pat. No. 4,854,496 to Bugle.
Prior cementless femoral components utilized one range of pore size. The extent of the porous coating was varied from 1/3 proximal coated (FIG. 1) to 4/5 proximal coated (FIG. 2) and even fully porous coated. Clinical investigation has determined that the more extensively porous-coated implants have the better result in terms of pain relief. It is, therefore, advantageous to maintain an extensively porous-coated implant in the new invention. It is not, however, a requirement to maintain bone ingrowth through this extensive porous implant. In fact, it is well known that it is difficult to design and obtain bone ingrowth through this extensive porous implant. In fact, it is well known that it is difficult to design and obtain bone ingrowth, at all. In some cases where there is no bone ingrowth, successful clinical results can be obtained entirely by a stable fibrous encapsulation. A priori, it is reasonable to assume that the naturally found normal physiologic loading of the proximal femur is the ideal objective to be obtained by an artificial implantation. A combination of maximizing bone ingrowth proximally with a diminishing amount of bone ingrowth through the middle zones of transition to a distal zone where fibrous tissue ingrowth is encouraged would serve to load the bone in a more physiological manner.
It was in light of the foregoing that the present invention was conceived and has now been reduced to practice.